The invention relates to solid acceptors suitable for removal of sulfur oxides from gases containing the same and to a process for their preparation. More particularly the invention relates to copper-containing solid acceptors for sulfur oxides which combine the desirable properties of superior stability and activity with a very high capacity for removal of sulfur oxides out of waste gas streams containing the same, their preparation and use in sulfur oxide removal processes.
By the term "acceptor" as herein employed is meant a solid substance which is capable of binding a gas or a gaseous compound either physically or chemically. Such acceptors may comprise a carrier on which one or more metals and/or metal compounds have been deposited. For example, copper and/or copper oxide supported on alumina is very suitable for the removal of sulfur oxides from gases under oxidative conditions. The sulfur oxides-- including both sulfur dioxide and sulfur trioxide-- are bound by the acceptor as sulfate, thus freeing the gases from sulfur oxides thereby permitting its discharge into the atmosphere without giving rise to air pollution. The loaded acceptor can subsequently be regenerated with a reducing gas which produces a regeneration waste gas rich in sulfur dioxide. This sulfur dioxide-rich gas can be used for the preparation of sulfuric acid or elemental sulfur. The regenerated acceptor is capable of binding fresh quantities of sulfur oxides under oxidative conditions.
According to U.S. Pat. No. 3,770,647 (common assignee) acceptors of the above-mentioned type can be prepared from carriers obtained by mixing aluminum hydroxide or alumina with a suitable binder such as clay, and forming the desired carrier bodies by drying and calcining the resultant mixture at temperatures above 780.degree.C. The finished acceptor is then obtained by depositing on the carrier a metal capable of accepting sulfur oxides by impregnation with a solution of the metal compound. Acceptors prepared in this manner offer the advantage of having a very high physical and chemical stability. Such properties are highly desirable, particularly for acceptors which are subjected to the action of both reducing and oxidizing gases at high temperatures. However, a disadvantage of the finished catalysts prepared in this manner is that they generally show a lower activity than catalysts or acceptors based on alumina-containing carriers which have not been calcined at temperatures above 780.degree.C.
U.S. Pat. No. 3,776,854 (common assignee) describes an improvement on the above-mentioned process for preparation of copper-containing acceptors wherein acceptors having a high initial activity in addition to a good chemical and physical stability are prepared by impregnating alumina-containing carriers, which have been calcined at a temperature above 780.degree.C with a solution of a compound of magnesium, aluminum, titanium and/or zirconium prior to or simultaneously with their impregnation with a solution of a copper compound. However, despite the combination of desirable properties exhibited by acceptors prepared according to this process, they still are not considered optimum since their capacity for removal of sulfur oxides- i.e., quantity of sulfur oxides adsorbed per a given quantity of acceptor- falls somewhat short of the theoretical capacity for an equivalent quantity of active acceptor material.